Laboratory evaluation of thyroid function: Dilemmas and pitfalls

Among all endocrine disorders, thyroid dysfunction is possibly most common endocrine disorder barring obesity. This implies that thyroid function tests (TFT) are routinely ordered for laboratory test for its evaluation. Furthermore, recently laboratory values of thyroid function, thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3), have gathered importance in view of rapidly changing cut-offs for treatment of thyroid disorders during pregnancy. Most of the times, interpretation of TFT is easy, indicating euthyroidism (normal FT4 and TSH), hypothyroidism (low FT4 or FT3 with high TSH), or thyrotoxicosis (high FT4 or FT3 with low TSH). However, the normal ranges reflect two standard deviations around the mean. Hence, 2.5% of the population may show minor abnormalities on both side of normal range in spite of being euthyroid. Sometimes interpretation becomes difficult when there is an alteration in relation between thyroid hormones and TSH. These pitfalls in investigations will cause dilemma in physicians and patients mind alike. Problems in hormonal evaluation can be preanalytical, analytical, and postanalytical. In an ambulatory patient, TFTs have limited preanalytical interferences such as age, pregnancy, medications, genetic mutations, systemic diseases, and critical illnesses. Analytical errors occur due to heterophile antibodies and macro-TSH. Postanalytical errors include wrong entry of the result, mistakes in the units of the parameter checked and failure to identify the normal data.

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